hello,

i have one question here. when the distributed generation has been islanding due to the fault, the power imbalance is occur, then by means of load shedding the islanding location can continue operate. what is the responds of the generator just after the load shedding? thank you....

 

It seems you are asking what happens when a generator and some load (the "island") are isolated from a larger grid. For example, let's say a generator and it's prime mover (a turbine or a reciprocating engine, for example) are rated at 20 MW, and the load of this small "island" that can be isolated from the grid and powered by this generator and it's prime mover is 25 MW. So, when this island and it's generator are isolated from the larger grid some load has to be shed (automatically disconnected) in order not to exceed the rating of the generator's prime mover. In this example, a total of 5 MW of load would have to be disconnected from the isolated island in order not to exceed the rating of the generators' prime mover if it were to be operated at rated frequency when isolated from a larger grid.

In order for a prime mover and it's generator to power a load at a constant frequency the prime mover needs to be operated in such a way that it is controlling frequency in response to changes in load. This is normally called Isochronous Governor mode, or Isochronous Speed Control mode. And a prime mover and its generator can only produce power at rated frequency up to the rating of the generator's prime mover.

When connected to a larger grid in parallel with other generators and their prime movers the normal mode of operation for the prime mover governor (control system) is Droop Speed Control mode, because some other "entity" is controlling the frequency of the grid. And, when connected to a larger grid in parallel with other generators and their prime movers a generator can only produce power up to the rating of its prime mover.

So, when a prime mover and generator are suddenly disconnected from a larger grid and are to be provide power to a load (the island) independent of the grid, the prime mover's governor is usually switched to Isochronous Speed Control mode. And, if the load of this small island exceeds the rating of the generator's prime mover then some of the load must automatically be disconnected, and this is referred to as load shedding.

Usually, there is a contact on the breaker that connects the island to the larger grid that serves to tell the prime mover governor to switch from Droop Speed Control mode to Isochronous Speed Control mode *and* to initiate load shedding to reduce the load below the generator prime mover's rating.

Depending on how fast the load is shed when the grid tie breaker opens and also depending on how fast the prime mover's governor (control system) can react to the change in load, what usually happens if the island load is initially greater than the prime mover's rating is that the frequency will decrease. Once the load of the island has been reduced to at least the rating of the generator prime mover and the prime mover's governor has responded to the change in load, then the frequency should return to rated and remain at rated as long as the load of the island does not exceed the rating of the generator's prime mover.

As long as the island load is not allowed to exceed the rating of the generator's prime mover it should be able to respond to any change in load and still maintain rated frequency. That is, the prime mover's control system (the governor) should be able to respond to any change in load up the rating of the prime mover and still be able to maintain frequency.

The generator produces power at a frequency that is proportional to the speed at which it's rotor is being turned by the prime mover (the turbine or reciprocating engine). When operating a small island of isolated load, the amount of load must be less than the rated power of the prime mover driving the generator, or else the frequency will not remain at rated.

It's not the generator that's controlling frequency or the amount of load, it's the control system (the governor) of the generator's prime mover that's controlling the frequency (when operating in Isochronous Speed Control mode) or the load (when operating in Droop Speed Control mode in parallel with other generators and their prime movers). A generator is just a device for converting torque (from a prime mover) into amperes. Those amperes can then be transmitted over wires to motors which convert the amperes into torque. (Lighting is a way to convert amperes into heat, heat so hot that it produces light.) So, the load of a generator is proportional to the amount of torque being produced by it's prime mover. And that torque is a function of the energy that's flowing into the prime mover (fuel or steam or water or wind).

The explanation above presumes a single generator driving a load independent of a larger grid.